fix: group tree view by process name instead of PID

[productdevbook]

Summary

• Changes tree view grouping from PID-based to process name-based
• Two node servers on different ports now appear under one "node" group instead of two separate groups
• ProcessGroup model updated: id is now the process name (String), added pids: [Int] array
• PID display shows single PID or "N PIDs" count with tooltip listing all PIDs
• Updated all grouping locations: PortTableView, MenuBarView, MenuBarPortList, PortGroupingService

Fixes #85

Test plan

• Start two servers with the same process name (e.g., two node instances) on different ports
• Verify they appear under one group in tree view
• Verify expanding the group shows all ports
• Verify PID column shows "2 PIDs" with tooltip
• Verify menu bar tree view also groups correctly
• Verify single-PID processes still show the PID number

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